Download The Pons - Dentistry 09

The Pons
WED.26/4
Lab neuro # 8
The Pons is anterior to the cerebellum and connects the medulla
oblongata to the midbrain , it is about 1 inch long.
Anterior surface :
Inside the pons we have the middle cerebellar peduncle which connects
the cerebellum with pons.
There is a shallow groove in the midline of anterior surface , the basilar
groove, which lodges the basilar artery.
On the anterolateral surface of the pons , the trigeminal nerve emerges
on each side
Each nerve consist of a smaller ,medial part ,known as the motor root,
and a larger, lateral part, known as the sensory root.
In the groove between the pons and the medulla ablongata there emerge,
from medial to lateral, the abducent , facial, and vestibulocochlear
nerve.
These three nerves are at the junction between medulla and pons
Note :
multiple cranial nerves come out from pons and as we know each cranial
nerve have anterior and posterior roots .
***{while studying this sheet you must see the pics in the book & sildes}
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Posterior surface:
The posterior surface is limited laterally by sup cerebellar peduncles and
is divided into symmetrical halves by a median sulcus. Lateral to this
sulcus is the medial eminence, which is bounded laterally by a sulcus
the sulcus limitans.
The inferior end of the medial eminence is slightly expanded to form the
facial colliculus.
Internal structure of the pons:
* the pons is commonly divided into a posterior part, the tegmentum
(mostly contains nuclei), and an anterior basal part (mostly contains
fibers)
* the structure of the pons may be studied at two levels :
(1) level of facial nucleus (lower pons)
(2) level of trigeminal nuclei (upper pons)
* trapezoid fibers separate the tegementum from the basal part
* Level of lower pons
(1) Tegmentum:
Medial lemniscus is situated in the most anterior part of the tegmentum.
At this level we can see:
- The 4th ventricle
- Superior peduncle (more prominent at higher level) which connects the
cerebellum with midbrain.
- Part of the inferior peduncle
- Level of facial folliculus which consists of 2 motor nuclei (nucleus of
abducent nerve & nucleus of facial nerve)
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Facial nucleus lies posterior to the lateral part of the medial lemniscus.
The fibers of facial nerve wind around the nucleus of the abducent
nerve, producing the facial colliculus. The fibers of the facial nerve then
pass anteriorly between the facial nucleus and the superior end of the
nucleus of the spinal tract of the trigeminal nerve.
NOTE:
Facial nerve lies between medial lemniscus & nucleus of the spinal tract
of the trigeminal nerve (which is the extension of Substantia gelatinosa
which extend ton end at the lower pons).
- medial longitudinal fasciculus (MLF) :
Contains ascending and descending fibers near the midline.
It is the main pathway that connects vestibular and cochlear nuclei
controlling the extraocular muscles (occulomotor, trochlear, and abducent
nuclei) for the movement of head, neck, and eyes.
- medial vestibular nucleus:
Situated lateral to the abducent nerve and in close relationship to the
inferior cerebellar peduncle.
- trapezoid:
Is made up of fibers derived from cochlear nuclei and the nuclei of the
trapezoid body.
NOTE:
Post view
Vestibular and cochlear nuclei are both sensory nuclei
Vestibular complex is made of 4 nuclei ( sup+ inf+ med+ lat) in each
section we can see part of these nuclei or none it depends on the section
Ex: in the lower section we can see med + inf
The ascending part of the MLF connects vestibular and cochlear nuclei
Medial longitudinal fasciculus (ascending MLF)
Medial vestibulospinal tract (descending MLF)
Only remainents of cochlear nuclei can be seen in lower Pons.
(2) Basilar part
(Mostly containing fibers )
It contains small masses of nerve cells called Pontine nuclei
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The axons of these cells give origin to the transverse fibers (transverse
pontine fibers) of the pons which crosses the midline and intersects the
corticospinal and corticonuclear tracts breaking them into small bundles.
This connection forms the main pathway linking the cerebral cortex to the
cerebellum:
* Corticospinal tracts  doesn’t form pyramids like in the medulla
because it doesn’t project to the surface it only passes inside.
* Corticonuclear tracts  the same as the corticospinal but it reaches the
cranial nerves.
* abducent nerve  abduction of the eye
Facial nerve  muscles of the face
* Level of upper pons
Same as the lower pons have :
-anteriorly  basal part
-posteriorly  tegmentum
Always post to medial lemniscus  spinal lemniscus is present
spinal lemniscus have part of the spinothalamic tract and the other part
tectothalamic tract.
* The Superior peduncle is larger in this section than in the lower pons
but not larger than in the midbrain
Inferior peduncle is not present here, only in medulla oblongata & pons.
Middle peduncle is present in basal part and more prominent because it
goes to the pons & cerebellum
*what makes this level special is the presence of two trigeminal nuclei:
(1) Motor trigeminal nucleus
Medially, smaller, within reticular formation
(2) Sensory trigeminal nucleus
Laterally, larger, extension to the spinal trigeminal nucleus
These give rise to the 5th cranial nerve (trigeminal nerve) which is a very
important nerve.
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* Pontine nuclei receives afferent fibers from cortex (temporal lobe,
frontal lobe) called frontopontine and tempropontine fibers it relay and
synapses there. Then it sends transverse pontine fibers (transverse
pontocerebellar fibers) to the middle cerebellar peduncle then to the
cerebrum.
This is the main pathway that sends information from cerebrum to
cerebellum.
* The corticospinal tract passing through this area won’t bulge to the
surface forming pyramids as we said why???
Because the transverse fibers while passing through this area it will cause
the break down of corticospinal bundles preventing the aggregation of
these fibers and so prevents forming the pyramids.
GOOD LUCK ALL : D
Done by: Razan Amayreh
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